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Introduction to Polymer Science and Technology Polymer processing3.3.5 Coating systems Paintsare used for decorative purposes but more importantly to protect and preserve materials. A liquid coating system consists of: polymer/resin (the binder) + solvent + pigment and extender. Pigment is the obliterating component because of its high refractive index compared with the binder and promotes colour. The polymer (or the resin) binds the pigment particles together as well as causing them to adhere to the substrate. The polymers employed as binders are usually solids or high viscosity liquids and require thinners (solvents) to give lower viscosities suitable for spreading. Drying and hardening processesfor the applied coating are air drying or heat drying and hardening. In air dryingthe solvent is eliminated from the applied paint by evaporation, and the film of coating is hardened by cross-linking reactions under the activation of the atmospheric oxygen. Whereas in heat drying and hardening,heat accelerates not only solvent evaporation but also cross-linking reactions. Paints that require heat for drying and cross-linking are known as curing or stoving paints. Coating techniques: Dipping: the component is heated in an oven to a temperature such that when it is dipped, the polymer adheres to the hot metal, melts, flows, and fuses into a coherent coating. The coated article is placed back into the oven, after dipping, to ensure complete melting and fusion of the polymer. Main stages in the dip coating are 1) metal preparation (e.g., degreasing, shot blasting and cleaning); 2) pre-heating (e.g., 230 and 400 °C depending upon the coating thickness required and the coating 3) dipping; 4) melting and fusion or curing. There are other coating techniques, e.g., a variety of spraying methods, hot metal foil stamping, and chemical and physical vapour deposition (PVD involves condensation of evaporated atoms on the substrate in a vacuum chamber, in the case of CVD a chemical interaction occurs between gases and substrate under heat to form a film on the substrate). Some of the techniques use an electric field: electrostatic and electrophoretic coating. Electrostatic coating involves spraying of an electrically grounded part with charged polymer powder. In Electrophoresis (also known as electrocoating/electroplating, used for coating a plastic with metal) the part is submerged in an immersion bath, and the process involves the motion of charged polymer (or ceramic/ metallic) particles through the liquid suspension to the electrode of opposite charge, which is the substrate/ part to be coated. The dipping process is facilitated with fluidised-bed coating (Figure 3.56): a gas is passed upwards through a vertical column containing polymer powder. At some particular gas velocity, the weight of the particles will be slightly less than the buoyant forceof the gas, and the powder becomes fluidised. Dipping objects into fluidised powder is much easier than dipping into stationary powder. Plastics powders commonly used are PEs, acrylics, nylons, PP and PVC. Introduction to Polymer Science and Technology Polymer processing Powder particle size range from a diameter of 30 to 250 u,m: Particles greater than 200 u,m are difficult to suspend and less than 30 u,m generate too much dust from the top of the bed. The process does not involve the use of any solvent and is a very efficient method, utilising almost 100% of the coating material. The technique achieves layer thicknesses of 250 to 500 u,m; even layers of greater than two millimetres are possible in a single step. heated spoon
Figure 3.56Illustration of the principle of fluidised bed coating Introduction to Polymer Science and Technology Polymer processing 3.4 Self-assessment questions 1. Indicate if the viscosity of a polymer decreases with increases in (a) shear strain rate, (b) molecular weight, (c) 2. Does MFI of a polymer increase or decrease with increasing viscosity? 3. How are weld-lines caused in injection mouldings and how do they affect the quality of the components? 4. Indicate true or false: weld lines become a source of weakness because polymer chains diffuse very slowly. 5. Cite factors which determine the choice of fabrication techniques for polymers. 6. What two parameters are used in rating an injection moulding machine? 7. In moulding, what is the purpose of cold-wells in a mould? 8. Excess flashing could be the result of
a) material being too hot b) mould being too hot c) injection pressure being too high d) all of the above conditions.
9. Why is it important to have the sections of a moulding as uniform in thickness as possible? 10. Distinguish between the screw types for the effective extrusion of Nylon 6,6 and polyethylenes. 11. On a standard extruder screw, there are three sections - what are they called? 12. What is the purpose if the tubes or passages under the surface of the feed throat? 13. What will probably happen if plastic melts too early and sticks to the screw in the feed zone? 14. Indicate two important microstructural characteristics for polymers that are considered for fibre production. 15. Why is the inside diameter of the calibrators (sizing rings) often bigger than the outside diameter of a tubular 16. Die swelling occurs because of
a) attempting to extrude a product at too fast a rate b) the chains become completely disentangled at high shear rates and expand when they re-entangle c) relaxation of shear oriented molecules d) the pressure of the polymer melt expands the die.
17. What processing conditions must be met in the production of PET bottles for soft drinks? 18. What processing method would you use to make a rigid plastic pipe that can be laid easily? 19. Which of the following processing methods would you use for compounding a polymer with colorants and
a) injection moulding b) thermoforming c) single-screw extrusion d) twin-screw extrusion e) transfer moulding.
20. Indicate true or false: PMMA glazing for aircraft windows is biaxially stretched because this encourages 21. Indicate the relationship between the melt temperature and the height of the frost line in blown-film production. 22. Moulds for blow moulding can be made using aluminium whereas moulds for injection moulding are usually Introduction to Polymer Science and Technology Polymer processing 23. What processing method(s) would you use to make disposable plastic plates? 24. Describe vacuum forming/thermoforming processes. What are typical thermoformed plastic products? 25. Calculate the length x width x thickness of a blank sheet needed to produce a rectangular container of Answer: 104 x 54 x 0.32 cm. 26. What processing method would you use to make large, hollow polyethylene playground items? 27. What processing method would you use to make a plastic traffic cone? 28. Describe a method for producing a rotational moulded part consisting of two solid layers of different materials. 29. Describe the circumstances when transfer moulding would be a better choice than compression moulding for 30. Describe the injection moulding process and distinguish between the injection moulding of TP and TS polymers. 31. Guess what the following pattern on the floor is, how was it produced, and relate it to one of the topics covered
32. Describe the methods of foaming/expanding plastics. 33. Describe the fluidised-bed coating technique, outlining the parameters which influence the quality of the coating. 34. Distinguish between electrostatic and electrophoretic coating. 35. Identify and briefly describe a method for coating a plastic with metal. 36. Injection moulding scrap from the sprue and the runner system can be reground and used again for processing, Introduction to Polymer Science and Technology Microstructure 4 Microstructure "Measure what is measurable, and make measurable what is not so." Galileo Galilei,1564-1642. Galileo would be proud of the scientific and technological progress in telescopes and microscopy that have enabled amazing measurements to be made at the scale of heavenly bodies, which was Galileos main field of activity, but also at the scale of atoms. In this chapter some measurements associated with the morphological and microstructural features of polymeric materials will be briefly outlined. Polymers consist of chain like molecules, where thousands of monomers (repeat units) are strung together. The arrangement of these repeat units with respect to each other is important both in thermoplastic and in thermosetting polymers. In thermoplastics the arrangement dictates whether the polymer is crystalline (orderly) or amorphous (disorderly), and in thermosets, particularly in elastomers, it also controls the propensity of the polymer (in unvulcanised state) to be crystalline or not and, therefore, its suitability for making rubber. Introduction to Polymer Science and Technology Microstructure 4.1 Stereoregularity Free-radical polymerisation of conjugated 1, 3-diene monomers such as butadiene, isoprene and chloroprene can produce sequences of repeat units of cis- and trans-configurations. Configurations (cisand trans)describe the arrangements of identical atoms or groups of atoms around a double bond in a repeat unit, e.g., cis- and trans-polyisoprene (Figure 4.1). In cis configuration the double bonds and these groups (CH3) are on the same side of the chain, and in trans configuration they are on opposite sides or across from one another (Figure 4.2). These configurational isomers are spatially fixed and, unlike conformations, cannot be switched from one to the other by rotation about covalent bonds. Ziegler-Natta polymerisation can produce almost 100% cis- or trans-1, 4-polymers from butadiene and isoprene monomers. In general, cis-polymers show a lower T and Tm values than the trans polymers, as in polyisoprenes and polybutadienes, since regular (symmetrical) structure of trans polymers enable crystalline formation. Cis-1,4-polyisoprene (natural rubber) do not normally crystallise unless highly strained and the molecules coil rather than remain linear, which gives rise to long-range rubber elasticity. cis-polyisoprene ñ=ñ trans-polyisoprene Figure 4.1Cis-1,4-polyisoprene and trans-1,4-polyisoprene
Figure 4.2 Geometricisomers: (a) cis-1,4-polyisoprene (natural rubber, e.g., from the tree Hevea Brasiliensis) and (b) trans-1,4-polyisoprene (gutta percha) (source: Weaver & Stevenson 2000) Stereoregularity(tacticity) refers to spatial isomerism in vinyl polymers and describes the arrangement of side groups around the asymmetric segment of vinyl-type repeat units, (- CH2 - CHR -). Consequently, three different forms of polymer chain results in thermoplastics: atactic, isotactic and syndiotactic. Figure 4.3 shows the regular arrangement of the side group R in a simple vinyl polymer: in isotactic form all side groups on the same side of the polymer chain and in syndiotactic form side groups alternate regularly on either side of the chain. Atactic form describes the random attachment of the side groups about the back-bone chain. Stereoregularity influences the ability of a polymer to crystallise and also the degree of crystallinity and, in turn, significantly influences properties. For example, polystyrene has large Introduction to Polymer Science and Technology Microstructure phenyl groups randomly distributed on both sides of the chain. This random positioning prevents the chains from aligning and packing together with sufficient regularity to achieve any crystallinity so the atactic PS is completely amorphous. However using metallocene catalysts an ordered syndiotactic PS can be produced, which is highly crystalline with a Tm of approximately 270 °C.
Figure 4.3Illustration of tacticity in polymer molecules 4.2 Morphology in semi-crystalline thermoplastics The crystalline structure of semi-crystallineTPs comprises unit cells(dimensions < lnm) and lamellae(«10-30 nm thick platelets which are formed by an orderly packing of folded chain segments). Lamellae grow from nucleiin a radial fashion into a larger structural unit, the spherulite(« 0.5-100 |am radius) (Figure 4.4). Figure 4.5 shows polypropylene spherulites of about 100 |am radius grown under controlled conditions on a microscope hot stage. But in real production situations the spherulites in thermoplastic films are imperfect in shape and much smaller in size (0.5 to 8 |am). Their size depends on production parameters such as the melt temperature, the rate of cooling/solidification, etc. Spherulite size and its uniformity significantly influence mechanical and optical properties. The interrelation of 'production parameters-spherulite size-material property' makes the on-line measurement of spherulite radius a worthwhile pursuit. An attempt was made by Akay & Barkley (1984), using the small-angle-light-scattering (SALS) technique. The principles of this technique are shown in Figure 4.6 and its adaptation for an on-line measurement of spherulite radii of clear film extrudate is illustrated in Figure 4.7. Introduction to Polymer Science and Technology Microstructure
lamellae Figure 4.4Illustration of spherulites and lamellar fibrils
Figure 4.5An optical micrograph of PP Spherulites Date: 2015-12-11; view: 1043
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spherulite